Activists rally on Capitol Hill during the March for Science April 22, 2017 in Washington, DC. Thousands of people joined a global March for Science with Washington the epicenter of a movement to fight back against what many see as an "assault on facts" by populist politicians.Brendan Smialowski — AFP/Getty Images

Activists rally on Capitol Hill during the March for Science April 22, 2017 in Washington, DC. Thousands of people joine

See The March For Science Protests Around The World

On April 22, 1970, nearly 20 million Americans (including members of both houses of Congress) came out to march in various cities to protest the spread of pollution, wildlife extinction, anti-earth behavior by corporations, and more. The event eventually helped start an environmental movement that led to the creation of the Environmental Protection Agency and the passage of a wide range of legislation and regulations designed to protect the environment. This Saturday, scientists and researchers added a new page to that legacy with the March For Science.

The activists who sponsored that 1970 protest—the first Earth Day—called it "a day to challenge the corporate and governmental leaders who promise change, but who shortchange the necessary programs." In 1990, the day became a global event, and over 200 million people in 141 countries joined in the marches to fight against environmental ignorance.

The present day has more in common than you might expect with the era of the first Earth Day. Although there's overwhelming scientific consensus behind the concept that human behavior is affecting the climate, most of the world and the U.S. in particular is still dealing with climate change deniers. Most people are still dependent on fossil fuels. And 2016 was the hottest year on record. Still, last year, environmentalists appeared to score a victory the biggest global agreement to address and combat climate change was signed on Earth Day. Then-President Barack Obama called it "the best possible shot to save the one planet we've got."

The Apollo 17 crew caught this breathtaking view of our home planet as they were traveling to the Moon on Dec. 7, 1972. It's the first time astronauts were able to photograph the South polar ice cap. Nearly the entire coastline of Africa is clearly visible, along with the Arabian Peninsula.Courtesy of NASA

The current President has other priorities. Talking on MSNBC's "Morning Joe" in 2015, then-candidate Donald Trump said, "I think one of the dumbest statements I've ever heard in politics— in the history of politics as I know it, which is pretty good, was Obama's statement that our No. 1 problem is global warming." Trump has also tweeted that climate change is a hoax created by China. He appointed Scott Pruitt, who has opposed the EPA and has filed numerous lawsuits challenging environmental regulations, as head of the EPA, and has rolled back many of Obama's regulations.

"Nearly five decades later, the EPA – and science itself – are under assault," wrote Denis Hayes, the original organizer for the first Earth Day in a message on the Earth Day Network website.

But this Saturday's events were set in motion by one recent trend, in particular: Trump's actions in silencing scientists. An uproar occurred when the Trump administration put a "gag order" on multiple federal agencies, and started taking down climate change informational pages from those agencies' websites. Multiple rogue Twitter accounts started popping up in an effort to make sure that climate-change related facts from those agencies were shown to the public.

After the success of the Women's March on Washington, the day after Trump's inauguration, an idea was launched on Reddit for there to be a "Scientists' March on Washington." One thing led to another, and within a week a Facebook page and Twitter handle were created, with more than a million followers, as well as a website; eventually the event took shape, with the Earth Day Network as one of its hosts. Around 100 groups, including the American Federation of Teachers, Girls Who Code, and the New York University have partnered up with the march on the National Mall.

"Currently, there is an anti-science, anti-expert movement afoot in the U.S. and around the world" Bill Nye, an honorary co-chair for the march, told Fortune last month. "We will use our feet to remind our leaders that science drives our economy and is a key to our future."